Chapter 4 of the Cisco Service Portal Designer Guide has a ton of information on portlet design. It's not a step-by-step guide but it should tell you everything you need to create a Hello World example and much more.
Here are the chapter sections:
Chapter 4, Portal Manager
- Overview, page 4-1
- Portal Designer, page 4-3
- Content Portlets, page 4-6
- Reserved Portlets, page 4-12
- Custom Content, page 4-15
- HTML and JavaScript Portlets, page 4-17
- JSR Portlets, page 4-21
- Portal Pages, page 4-25
- Portal Settings, page 4-31
- Reference Data, page 4-36
- An End User’s View of the Portal, page 4-46
- Importing and Exporting Portal Content, page 4-52
- Portal Access Control, page 4-56
It's pretty easy to create simple portlets that list core entities, service items and standards. If you want to go beyond that then focus on the HTML and JavaScript Portlets section. That section contains info on how to use the Portal REST API and EXTJS grid functionality.
You can find quick links to other useful Portal documentation on my recent post here.
